1. Field of the Invention
This invention relates to ethylene polymer plastic compositions useful for producing extruded or molded polyethylene products having improved transparency.
2. Description of the Prior Art
Polyolefin resins such as polyethylene have been used for a number of years for the preparation of products such as packaging materials and containers in the form of films, sheets or molded articles.
Unlike high-pressure low-density polyethylene, where the optical properties are governed primarily by rheological factors, the optical properties of narrow molecular weight distribution ethylene polymers are controlled primarily by crystallization effects, i.e., spherulite size. Heterogenous nucleating additives can improve ethylene polymers opticals by increasing the nucleation rate and hence the overall rate of crystallization, thereby generating smaller spherulites.
Heretofore extremely pure dibenzylidene sorbitol (DBS) (commercially known as EC-1 and available from EC-1 Chemicals Industries, Osaka, Japan or from Milliken Chemical Co., USA,) has been used as a nucleating agent to improve the optical properties of polyethylene products.
Thus, according to U.S. Pat. No. 4,016,118, issued to Kenzo Hamada et al on Apr. 5, 1977, there is provided a polyolefin plastic composition having improved transparency and reduced molding shrinkage, which consists essentially of at least one polymer or copolymer of an aliphatic monoolefin and dibenzlyidene sorbitol, the proportion of the dibenzylidene sorbitol being 0.1 to 0.7% by weight based on the total weight of the composition.
However, under some fabrication operations and conditions, even high levels of DBS may not improve the transparency (reduced haze) of Ziegler type catalyzed linear lower density polyethylene (LLDPE) since a critical concentration is required. Moreover, even in the case where DBS is effective, high concentrations are required. Since DBS is an expensive material, reduction of the concentrations needed to produce desired results is of commercial significance.
Moreover, the use of DBS to enhance the optical properties of LLDPE is not entirely satisfactory from a practical commercial standpoint since the effectiveness of DBS as a nucleating agent can be seriously impaired in the presence of the catalyst residues and/or by-products generated during the LLDPE resin-forming process or during the fabrication of the desired product.
Optical properties of extruded films produced from Ziegler-Natta catalyzed resins containing antioxidants can be improved with the addition of an acid acceptor to prevent surface distortions known as pin striping and gel streaking. The surface distortions are produced as a result of adverse interactions between catalyst residues and/or by-products, antioxidants and the resin.
Thus according to U.S. patent application Ser. No. 215,165 filed in the name of Michael A. Cowan and George N. Foster and assigned to a common assignee, the optical properties described above are improved by extruding a resin film forming composition which contains an antioxidant in the presence of an acid acceptor which reacts with the catalyst residue, the latter being suspected of cuasing optical degradation via the pin striping mechanism.
Other treatments disclosed in the prior art involve the addition of compounds to the polymer prior to fabrication in order to complex with the harmful components in the catalyst residue and deactivate them. Illustrative of these prior art treatments are those disclosed in Canadian Pat. No. 961,998, U.S. Pat. No. 4,013,622 and in U.S. Pat. No. 3,773,743. For example, U.S. Pat. No. 3,773,743 discloses a method for improving the stability and color of olefin polymers by deactivating their Ziegler-Natta catalyst residues. This method involves high temperature (190.degree. C.-250.degree. C.) processing with hydroxyl compounds (H.sub.2 O and primary alcohols) and with an organic base such as an alkyl amine; aryl amine; Li, Ca and Zn salts of carboxylic acids; trialkyl phosphites; and metal alkoxides. The concentration disclosed for the hydroxy source is in the range of 0.5 to 1.5 weight percent and a concentration of 50 to 2500 parts per million (ppm) is disclosed for the organic base. At the processing temperatures disclosed, some of the organic bases and the hydroxy sources would be volatile and cause foaming of the product if the processing were not done in a way to remove volatiles.
However, as mentioned previously, in processing LLDPE resins, clarity in the final product is determined by crystallization factors of the composition and hence control of the formation of pinstripes and/or other surface defects (gel streaking) by the above prior art methods is independent and fundamentally different from the techniques described herein.